[R-390] The R-725 and the DF story?
[email protected]
[email protected]
Mon, 08 Dec 2003 15:15:45 -0500
For some reason my last post was truncated. Let me simmarize that I believe that DF can be determined by measuring phase or time of aarrival differences from multiple receivers / and fixed antennas at known positions... usually in the form of collocated vertical elements in a constellation of some kind. If the 725 was used like this, there would probably be one dedicated to each of several antenna elements, then the IF outputs processed to measure phase differences and calculate a bearing to the transmitter. The original mechanical filters probably distort phase information (due to steep skirts/ringing) making precise/repeatable emasurement difficult - thus the change to the analog IF filters. Also, the multiple receivers and receive paths would have to be "calibrated" to account for differences in delay and phase response. So I suspect there were 3 or more receivers, with matched IFs, and one IF post processor doing the bearing calculations. Any thoughts?
----- Original Message -----
From: [email protected]
Date: Monday, December 8, 2003 11:16 am
Subject: Re: [R-390] The R-725 and the DF story?
> Attempting to answer John's initial question about reasons for the
> DF friendly IF deck. I'm not the world's expert in DF (although
> this has prompted some side reading today). Here's my
> understanding: Some DF systems employ fixed antennas (like Adcock
> arrays, they don't physically rotate), therefore the angle of
> arrival of the signal must be deduced by measuring relative phase
> differences or time of arrival at each of the antennas. This is
> akin to an interferometer. A system of this type using R-725's
> would employ several fixed position antenna elements at known
> spacings, with a separate receiver attached to each antenna. The
> IF outputs would go to some form of DF post-processor which would
> correlate the multiple replicas of the signal and derive
> phase/time difference of arrival. In theory, by knowing the phase
> differences at each antenna, and knowing the precise antenna
> spacing, the angle of arrival of the signal can be calculated.
> Nothing rotates. It's all in measu!
> ring phas
>
> ----- Original Message -----
> From: Dan Merz <[email protected]>
> Date: Monday, December 8, 2003 8:42 am
> Subject: Re: [R-390] The R-725 and the DF story?
>
> > John, I liked your typing into the wind, as you humbly put it.
>
> > This provokes
> > the idea that there may be more than one truth (= the best
> > exposition of the
> > known facts) about the R-725. I have excerpts from the
> "Direction
> > Finder Sets
> > AN/TRD-3, AN/TRD-23A and Direction Finder Sets AN/TRD-15 and
> > AN/TRD-15A"
> > TM-115825-231-24, August 1973, that pertain to the R-725. I
> > don't know what
> > else is in this manual outside of the parts I have on the R-725
> or
> > what the type
> > of direction finder equipment is involved. This information was
> > provided to me
> > by Tom when I became interested in modifying the 390 i.f.
> chassis
> > to fit my
> > 390a, which was completed. Maybe this will help, if someone
> > else can look at
> > the complete manual for details on the df equipment. Dan.
> >
> > John KA1XC wrote:
> >
> > > Since we're on the subject, I'd like to bring up a question
> > that's been on
> > > my mind about the reported purpose of the R-725 and its DF
> > friendly IF deck.
> > > Is DF use really the case, or is this just a story that gets
> > repeated?>
> > > While I've often read this explanation, I've never seen any
> > > documentation referring to what actual DF equipment or systems the
> > > R-725 was used with. I'm very familiar with the need for carefully
> > > characterizing the IF phase or group delay characteristics for
> > particular> applications, but I am unfamiliar with any needs for
> > DF. Maybe I don't
> > > understand this requirement but could there be another
> > explanation? The
> > > following is my reasoning.
> > >
> > > DFing to me means determining the location of an emitter
> > > >From what I know about the major HF DF systems used (such as
> > the giant
> > > Wullenwebber arrays that were deployed world-wide) they could
> be
> > simplified> into three parts:
> > >
> > > i) the antenna array which is used to receive and resolve the
> > emitter> bearing
> > > ii) the receiver which allows you to listen in and provides a
> > conditioned> IF output
> > > iii) the display processing equipment which takes the IF
> signal
> > and extracts
> > > the amplitude information and puts it in a form which can be
> > used to create
> > > a rotating polar display. This gives you the familiar DF scope
> > with the
> > > propeller shaped display indicating bearing.
> > >
> > > The whole point of this is that it is the emitters amplitude
> > which is being
> > > plotted against bearing, and I just don't see the emitters
> phase
> > components,> or the IF's, coming into the equation. I've also
> > heard stories told by DF
> > > and intercept operators of such centers being filled with
> racks
> > and racks of
> > > R-390's, R-390A's, and R-391's, but don't recall R-725's being
> > mentioned> much if at all.
> > >
> > > Perhaps I'm missing something in the above explanation, but I
> DO
> > have some
> > > applications in mind where linear IF phase would be handy.
> > >
> > > 1) Radio-location, defined as determining where the *receiver*
> > is located
> > > based on known transmitters.
> > >
> > > The transmitted signals contain precisely timed pulse
> > information which can
> > > be translated to distance, so having an IF that preserves the
> > phase (and
> > > therefore the timing) of the pulses is important. But there
> was
> > lots of
> > > specialized radiolocation receiving equipment built and sold;
> > why would you
> > > use an R-725 (plus other equipment), and why buy an expensive
> > receiver that
> > > covers all of HF when radiolocation utilizes lower frequency
> ground> > wave? This doesn't make much sense to me.
> > >
> > > 2) Data communication, involving something more complex than
> the
> > usual> multi-channel TTY, but have not seen any references to this.
> > >
> > > 3) ELINT, Electronic Intelligence gathering, a big time Cold
> War
> > activity.> This is where you are interested in the actual RF
> > signal itself so that it
> > > could be analyzed and information extracted, or so that it's
> > "signature"> could be determined.
> > >
> > > Back when the NSA was formed, they (and their various
> agencies)
> > started> analyzing all the foreign signals they could from DC to
> > light, and continue
> > > to do so today.
> > >
> > > Wide-bandwidth analog recorders had arrived on the
> > > scene and remote listening posts, planes, subs, and ships
> > hugging other
> > > countries borders were filled with all kinds of receiving gear
> > quietly> listening in and feeding these wideband recorders
> signals
> > straight from
> > > their IF outputs.
> > >
> > > Miles of tapes were routinely recorded every day, then rushed to
> > > centralized analysis labs for study. That is one of the main
> reasons> > surveillance receivers have IF signal outputs.
> > >
> > > This is one application where keeping all of an unknown
> > signal's amplitude
> > > and phase components intact would be the highest priority,
> since
> > the goal
> > > would be to record the desired signal in its original form
> with
> > the minimum
> > > distortion possible.
> > >
> > > For this special purpose the R-725 would fit the
> > > bill, the few hundred built could have been all that was
> needed.
> > ELINT was
> > > routine on VHF up through the microwaves, so why not HF too?
> > >
> > > Could HF ELINT have been the real purpose for this radio? The
> DF
> > story might
> > > be just that, a very believable cover story used to explain
> the
> > procurement> contract, brought to you by the cloak and dagger
> > folks that
> > > like to keep their real business quiet.
> > >
> > > Perhaps I've just been typing into the wind, but just maybe
> > there is someone
> > > reading who could chime in and perhaps shed some light on this.
> > >
> > > thanks,
> > > John
> > > KA1XC
> > >
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> >
> >
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